1. Field of the Invention
The present invention relates to an internal stress testing device for testing internal stress generated upon high speed electroplating.
2. Prior Art
Plating technology is widely used in various fields of our daily life. Particularly, in the electronics industry, high speed electroplating has been widely known for plating on lead frames of semiconductors such as integrated circuits.
High speed electroplating is carried out either by moving a plating solution at high speed under high current density or by splaying the plating solution at high speed. Such method ensures approximately a hundred times faster plating velocity compared with the conventional known plating method.
In this high speed electroplating method, various conditions for electrodeposition are required such as suitable plating bath composition, current density, agitation speed and the like. These conditions are kept optimal at the beginning, however, composition of the plating bath tends to change gradually due to its repeated use. Also, predicted and unexpected impurities are mixed into the plating bath. Plated films (plating deposits) to be made are affected by the change of the plating bath, and hence leading to deteriorated plated film quality in the end.
The optimal composition of the plating bath and the optimal conditions for electrodeposition are required so as to obtain the best plated films constantly. According to the change of the plating bath, addition of chemicals, removal of impurities, or adjustment of the various conditions such as current density is required as well as predicting the change in the plating bath composition. To this end, it is necessary to well understand the conditions of a fresh plating solution and the plating solution now in use.
Various factors should be considered for the optimal conditions of the plating bath. Measurement of internal stress of a plating deposit is one of the most important factors. An internal stress testing device utilizing a spiral contractometer is known as a testing device which can create similar plating conditions of the high speed electroplating method.
As shown in FIG. 6, an internal stress testing device 50 utilizing a spiral contractometer comprises a spiral-shaped test piece 51 fixed to a rotational axis 52 by attachment clamps 53, 54, a metal plate 55 for anode, and a plating tank filled with a plating solution. The test piece 51 and the metal plate 55 for anode are positioned within the plating tank, and they are connected to a power supply (not shown). When an electric current flows between the test piece 51 and the metal plate 55 for anode, a plating deposit is made on the surface of the test piece 51 and internal stress is generated at the plating deposit on the test piece 51. The test piece 51 is then rotated due to the internal stress thus generated. The internal stress at the plated film is measured by converting the rotation into the angle of torsion (.theta.) with the use of a transducer 56.
Internal stress (.sigma.) of a plated film is given by the following formula (1): ##EQU1## where, K: Spiral constant (mm N/deg), .alpha.: Angle of torsion (deg),
p: Spiral pitch (mm), PA1 t: Thickness of the spiral plate (mm) PA1 d: Thickness of the plating deposit (mm)